A number of devices using microwaves for indicating point-level of material in a storage container or vessel have heretofore been proposed and made commercially available. For example, U.S. Pat. No. 4,661,817 discloses an apparatus for measuring the distance to an object, particularly the distance to the surface of a liquid in a container. The basic concept involves transmitting microwaves toward the product surface and receiving reflected microwave energy from the surface. The reflected microwaves are analyzed to determine the distance that they have traveled. Knowledge of distance traveled and storage vessel height allows determination of product level. This can, for example, be achieved applying the well-known FMCW (frequency modulated continuous wave) technique or pulse-RADAR technique.
FMCW uses a linear modulated triangle or saw-tooth signal. It is obvious that the modulation time must be longer than the time of flight of the signal. The received signal has the same characteristics as the transmitted signal, but is delayed by the flight time. Both signals are mixed, thus yielding a frequency difference which in being directly proportional to the distance to be detected between antenna and object.
Pulse measurement systems are simple time-of-flight systems wherein short microwave pulses are transmitted and a reflected microwave signal is received. The distance is proportional to the time measured between sending and receiving the pulse. A commercially available pulse-RADAR sensor is, for example, the VEGA-Puls 64 FV device made by VEGA, Germany.
One accepted standard in the process control industry is the use of 4-20 mA process control loops. By this standard, a 4 mA signal represents a zero reading and a 20 mA signal represents a fill-scale reading. If the transmitter only requires sufficiently low power, it is possible to power the transmitter using current from a two-wire loop. This avoids the need for a four-wire setup, with separate power supply and signal transmission wires, and is therefore highly desirable.
Hitherto, an obstacle to this is the high-power requirement of most available microwave sources and receivers. U.S. Pat. No. 5,672,975 suggests a 4-20 mA two-wire level transmitter comprising a low-power microwave source and a low-power microwave receiver. It is not disclosed how such low-power components could be obtained, and U.S. Pat. No. 5,672,975 thus appears to be based more on a theoretical than a practical solution. In any case, all available low-power semiconductor transmitters are very expensive. Moreover, as a result of poor converting efficiency, they are characterized by exceptionally low transmission power, thus only yielding very weak reflected signals. To obtain a sufficiently exact measurement, it therefore would in effect be necessary, in many instances, to operate the level measurement system in a four-wire configuration rather than with a two-wire line. The additional wires are required to supply adequate operating power to the transmitting device, thereby sacrificing, however, the important benefits of a two-wire system.